Menopause evolution's way to end
moms-in-law reign

5 April, 2008

A new study has found that menopause,
which marks the end of menstruation,
has an “evolutionary” significance.

According to two British scientists,
menopause is “the evolutionary way of
solving the age-old tensions between
women and their dominant
mothers-in-law.” In their study,
published in the Proceedings of the
National Academy of Sciences, Dr
Michael Cant of Exeter University and
Dr Rufus Johnstone of Cambridge
University, have put forward the new
theory of how evolution shaped the
fertility of women. They also believe
that the new finding could give clues
to the genetic basis of premature
menopause and other diseases of low
fertility.

Most women lose the ability to
conceive at about age 50. It has
puzzled scientists why, after carrying
out their most important evolutionary
function, they not only go on to live
much longer but outlive men, too.

The most popular theory, according to
an article in Telegraph, is
that “menopause is nature’s way of
saying it is time to become a
grandmother, so women can look after
grandchildren in their fifties and
sixties, giving their hard pressed
daughters a break from motherhood to
allow them to become pregnant again.”

In tight-knit ancient communities,
there was often food to feed only one
woman’s offspring, and the
daughters-in-law won the fight for
resources.

Dr Michael Cant and Dr Rufus Johnstone
offer a new answer to this problem,
based on the realization that our
ancestors lived in small groups and
members of those groups had to compete
for food and other resources, which is
critical for raising the young.

The Telegraph quoted Dr Michael Cant
as saying, "The timing of reproductive
cessation in humans is best understood
as an adaptation to minimize
reproductive competition between
generations of females in the same
family unit. Simply put, it is in the
interest of mothers – and their genes
– to stop having children when younger
family members start to reproduce.”

"Humans,” according to Dr Cant, “are
unique among primates because there is
almost no overlap of reproductive
generations. In natural fertility
populations, women on average have
their first baby at 19 years, and
their last baby at 38 years; in other
words, women stop breeding when the
next generation starts to breed.”

The two British researchers argue that
there is a “mathematical logic” to the
menopause. Dr Cant says: “The point of
life is to pass genes down the
generations.

Looked at this way, it is in the
interest to mothers in law to give
their daughters-in-law a clear berth,
particularly in simpler societies
where extended families lived in one
place. An older female does best to
avoid competing because she is related
to her daughter-in-law’s offspring
(and therefore share’s an interest in
her reproductive success), but not
vice versa.”

“Despite vast differences in wealth,
resources and access to medicine,” the
researchers wrote in their study,
“women in all societies experience
menopause. This suggests that the
human fertility schedule is hard-wired
into our genetic makeup as a
consequence of our evolutionary
history, prior to more recent cultural
and technological advances.”

The study, they added, also helps to
explain why in some societies,
particularly in Africa and Asia, women
are required by social law to stop
having children when their first
grandchild is born.